1. Field of the Invention
This invention relates to a combustion state detection apparatus of an internal combustion engine for detecting a combustion state of the internal combustion engine by an ion current flowing as a medium of an ion occurring by combustion in the internal combustion engine.
2. Description of the Related Art
FIG. 7 is a block diagram showing a configuration of an apparatus according to the related art. FIG. 8 is a timing chart showing an operation thereof.
Numeral 1 denotes a control unit.
Numeral 1a denotes an ion current detection circuit for converting an ion current into a voltage value, including a current mirror and a resistor for ion current detection.
Numeral 1b denotes a mask circuit for eliminating a noise signal superimposed on the ion current at the time of turning on an ignition signal and after the completion of discharge.
Numeral 1c denotes combustion/misfire determination section for comparing a signal after mask with a predetermined reference level (Vref1) and making a combustion determination when the signal is larger than the reference level or making a misfire determination when the signal is smaller than the reference level.
Numeral 1d denotes an ignition signal generation section.
Numeral 2 denotes an ignition coil.
Numeral 2a denotes a primary winding of the ignition coil.
Numeral 2b denotes a secondary winding of the ignition coil.
Numeral 2c denotes a drive circuit including a preamplifier and a switching element.
Numeral 2d denotes a bias circuit including a capacitor for storing a bias voltage for ion current detection, a Zener diode for defining a bias voltage value and a diode for forming a secondary current path.
Numeral 3 denotes a spark plug.
By an ignition signal (signal A) outputted by the ignition signal generation section, the drive circuit energizes and breaks a primary current flowing through a primary winding of the ignition coil. At a time of the break, a high voltage occurs in a secondary winding of the ignition coil and a discharge is performed between electrodes of the spark plug and combustion of mixed gas inside a cylinder (not shown) is performed. The capacitor inside the bias circuit is charged by a secondary current flowing through a secondary winding during the discharge, and this charge voltage is applied to the spark plug after the completion of the discharge, and an ion current flows using an ion occurring by the combustion as a medium. This ion current is converted into a voltage by the ion current detection circuit in the control unit (signal B), and noise occurring at the time of turning on the ignition signal and after the completion of the discharge is eliminated by the mask circuit (signal C), and the combustion/misfire determination section makes a combustion determination when a signal level after mask is larger than a predetermined level (Vref1), or makes a misfire determination when the signal level is smaller than the level.
At a time-of combustion, a signal after mask at the time of misfire is indicated as shown in FIG. 8 and it can be determined whether or not the mixed gas has burned, but in the case of the misfire, a failure cause bringing about the misfire and a failure part cannot be identified. Also, in the case of failure of a detection system (ex. a break in wiring between the bias circuit and the ion current detection circuit), the signal after mask becomes equal to that at the time of the misfire. Because of this, when misfire detection is performed by the conventional apparatus and a misfire determination is made, there is a problem that it is necessary to inspect all the parts and detection systems having the possibility of causing the misfire for the purpose of repair and the number of man-hours of the repair is large.